Toxoplasma gondii's tropism for the CNS underlies the symptomatic disease it causes in developing fetuses, the immunocompromised, and, occasionally, the immunocompetent. As an intracellular parasite, Toxoplasma's ability to co-opt host cell function determines both the parasite's survival and its effects on the host. What is known about the Toxoplasma-host cell interaction comes primarily from in vitro studies in fibroblasts and immune cells. These studies have revealed that Toxoplasma commandeers host cell functions in part through the injection of effector proteins and that these proteins target boh universal and host-cell-specific pathways. While these studies established much about Toxoplasma-host cell biology, they will have missed host cell pathways specific to the CNS or only triggered during in vivo infection. These limitations are particularly relevant to the CNS, which is composed of multiple interacting cells types that maintain a baseline anti-inflammatory state. Given the vital nature of the CNS- Toxoplasma interface to clinical disease, understanding the CNS-specific effects of Toxoplasma is essential to eventually preventing symptomatic neurologic disease. To this end, we recently engineered Toxoplasma strains that inject Cre recombinase into host cells concomitantly with the effector proteins. By using these parasites to infect mice that express a green fluorescent protein only after Cre-mediated recombination, we are able to permanently identify and profile CNS cells injected with Toxoplasma effector proteins. Substantial preliminary data from this model strongly suggests that, contrary to existing dogma, Toxoplasma primarily interacts with neurons in vivo, which means that neurons are the major parenchymal CNS cell directly manipulated by Toxoplasma. The goal of this proposal is to determine how the neuron-Toxoplasma interface drives the establishment and outcomes of CNS infection. To this end, we will use our new mouse model in combination with two genetically divergent strains of Toxoplasma that cause different CNS outcomes and that we have engineered to inject Cre. Specifically, using cut- ting edge CNS imaging techniques that we have developed, we will determine if these Toxoplasma strains cause distinct CNS outcomes by infecting different CNS regions or co-opting different neuron subtypes (Aim 1). Additionally, to determine if these strain-specific CNS effects are secondary to differential manipulation of neurons, we will isolate and transcriptionally profile these Toxoplasma-injected neurons (Aim 2). The successful completion of this research will provide the first mechanistic insights into how neuron-Toxoplasma interactions drive CNS disease. Ultimately, such insights will offer us new avenues for developing CNS-specific or even strain-specific therapies that improve the outcomes of CNS disease.